CMSC 414 Computer and Network Security Lecture 10

CMSC 414Computer and Network SecurityLecture 10 Jonathan Katz

Administrative stuff • Office hours today • Exam review?

Paging • Similar to segmentation, but with fixed-size segments called pages • Addressing via (page, offset) • Avoids fragmentation problem…as well as “large offset” issue • Inefficient as program grows, since pages cannot be dynamically resized • “Re-paging” also causes potential security problems as data is shifted from one page to another • Also can be difficult to describe desired protection, since pages are no longer logical units

Best of both worlds? • Can combine paging with segmentation • Logical units, each broken into same-size pages • Advantages/disadvantages?

Access to general objects • Memory protection is only one example • Need a way to protect more general objects • Before we begin, some design principles…

Overview • Seminal article by Saltzer and Schroeder (1975) • Eight principles underlying design and implementation of security mechanisms • Linked from the course homepage

Key point I • Simplicity • Make designs/mechanisms easy to understand • Less chance of error

Key point II • Restriction • Minimize the “power” of an entity • E.g., only allow access to information it needs • E.g., only allow necessary communication; restrict type of communication allowed • Less chance of harm!

Principle 1 • “Principle of least privilege” • A subject should be given only the privileges it needs to accomplish its task • The function of a subject (not its identity) should determine this • I.e., if a subject needs certain privileges only to complete a specific task, it should relinquish those privileges upon completion of the task

In practice… • Systems are often not designed with the necessary granularity • E.g., “append” may not be distinct from “write” • E.g., in UNIX, no access controls for root • Anyone who can make backup files can also delete those files

Principle 2 • “Principle of Fail-Safe Defaults” • Unless a subject is given explicit access to an object, it should be denied access • I.e., the default is no access • E.g., a process reporting an error message should not try to expand its rights in an attempt to correct the error

Principle 3 • “Economy of Mechanism” • Security mechanisms should be as simple as possible… • …but no simpler! • Can simplify formal proofs of security (or even informal audits)

Consequences • If design/implementation are simple, less chance for error • Software testing is also simpler • Software interfaces especially suspect • Typically make assumptions about the input/output format of the other module • E.g., finger protocol: DoS attack by returning infinite stream of characters

Principle 4 • “Principle of Complete Mediation” • All accesses to objects should be checked to ensure they are allowed • OS should mediate any request to read an object --- even on the second such request by the same subject! • Don’t cache authorization results

Insecure example… • In UNIX, when a process tries to read a file, the system checks access rights • If allowed, it gives the process a file descriptor • File descriptor is presented to OS for access • If permissions are subsequently revoked, the process still has a valid file descriptor! • Insufficient mediation

Principle 5 • “Open Design” • No “security through obscurity” • Security of a system should not depend on the secrecy of its implementation • Of course, secret keys do not violate this principle!

Principle 6 • “Separation of Privilege” • (As much as is feasible…) a system should not grant permission based on a single condition • E.g., require more than one sys admin to issue a critical command, or more than one teller to issue an ATM card

Principle 7 • “Principle of Least Common Mechanism” • Minimize mechanisms depended upon by all users • Shared mechanisms are a potential information path, and should not compromise security • Also expose the system to potential DoS attacks

Principle 8 • “Psychological Acceptability” • Security mechanisms should not make access to the resource more difficult • If mechanisms are too cumbersome, they will be circumvented! • Even if they are used, they may be used incorrectly

Back to more specifics…

Back to specifics… • File protection as the running example • But everything said here is more generally applicable

Access control matrix • One central matrix indexed by all subjects and objects • Characterizes rights of each subject with respect to each object • Formally: set of objects O and subjects S • Matrix A contains an entry for every pair (s, o) • The entry contains the rights for s on o • Examples: read/write/execute/etc.

More complex access control • In general, “rights” may be functions • “Actual” rights depend on the system state • Equivalently, may depend on system history

Drawbacks… • Number of subjects/objects is very large • Most entries blank/default • One central matrix is modified every time subjects/objects are created/deleted or rights are modified

Directories • Can be viewed as storing the columns of the access control matrix with the appropriate subject • Each user has file directory, which lists files to which user has access and relevant access rights • Read, write, execute, “owner” - can change access rights on a file • Drawbacks • File directories can be large and difficult to manage • E.g., when a file is made public, all user’s directories must be updated! • Or, revoking all access to a file

Access control lists (ACLs) • Can be viewed as storing the rows of the access control matrix with the appropriate object • One list per object, showing all subjects with access and their rights • Possible to assign “default rights” to an object • Easy to make an object public • Example: access based on user, group, and compartment • Use of wildcards

Some design decisions • How fine-grained to allow ACLs? • E.g., user-level, group-level, or only public/private? • Granularity of rights (e.g., “append”?) • How to handle conflicts if two subjects give different permissions on an object • Disallow multiple owners • Allow access if any entry gives rights • Allow access only if no entry denies rights • Apply first applicable entry • Revocation?

Design decisions… • What is the default access level? • Two approaches • Apply ACL entry, if it exists; otherwise, apply default rule • I.e., ACL entries override default permissions • Augment the default permissions with those in the appropriate ACL entry • Example: default allows “read” and ACL entry states “write”

Capabilities • Some burden for implementing protection placed on the user rather than just the OS • Analogy: user has a “ticket” which grants access to an object • A capability is an unforgeable token giving user access to an object and describing the level of allowable access • Object owners can specify new types of rights

Two general approaches • Ticket is held by OS, which returns to the subject a pointer to the ticket • Ticket is held by the user, but protected from forgery by cryptographic mechanisms • How…? • Notencrypted as mistakenly claimed in book! • Two possibilities: ticket verified by the object or by the OS itself • Who holds the key in each case…?

Drawback • Does not really satisfy principle of complete mediation • Can add automatic expiration to mitigate this

CMSC 414 Computer and Network Security Lecture 10